The invention relates to a method for forming a semiconductor element enabling the detection of a fire with or without flame, according to which a phthalocyanine layer is applied by vacuum evaporation on a substrate, which layer is solidified in such a manner as to obtain acceptor and donor sites on the surface of the semiconductor element.
Such a method is known from the French patent application No. 2.383.440. According to the known method, a mixture of semiconductor powders comprising metallophthalocyanine is prepared. Those powders are deposited onto an insulating substrate by vacuum evaporation in order to form a film of semiconductor elements. The substrate is also completed by adding the necessary electrical contacts. The elements can then be mounted in an electric circuit for fire detection.
With phthalocyanines it is well known that certain active sites are carriers of a slightly positive electric polarity. Those acceptor sites are situated near the center of the phthalocyanine molecules at the level of the metallic atom. On the other hand, other sites are situated near the periphery at the level of the nitrogen atoms and are carriers of a negative polarity.
When the phthalocyanine layer is applied by vacuum evaporation, a drawback of the known method is that the active surface of the film forming the semiconductor layer corresponds to its real geometry, which considerably reduces the number of active sites, preventing the gas from reaching one of the active sites of the phthalocyanine molecules and thus detecting a fire without flame.
The U.S. Pat. No. 4,381,440 describes also the application of phthalocyanine by painting a powder dispersed in a solvent. When the semiconductor elements are obtained in such a manner, the two types of active sites are accessible in such a manner that a fire with and without flames can be detected. Unfortunately the powders undergo a sintering in time, which progressively reduces the access to the different types of sites. The sensitivity constantly reduces by the ongoing sintering process and finally the sensors become completely insensitive.
The international application WO 95/05595 also describes a manufacturing method of such a semiconductor element. According to this method the sensitivity of the detector to the one or the other type of fire is obtained by the choice of the phthalocyanine applied on the substrate.
The object of the invention is to provide a method for manufacturing a semiconductor element enabling destabilization of the crystallization of the films obtained by vacuum evaporation in such a manner as to enable combustion gases to reach the active donor sites as well as the acceptor sites of the phthalocyanine molecules and to reduce the loss of sensitivity that the sensors undergo in time.
To this end, a method according to the invention of a semiconductor element is characterized in that, during the vacuum evaporation, the substrate is cooled to a temperature between xe2x88x9230xc2x0 and 80xc2x0 C., preferably xe2x88x9220xc2x0 C. At this temperature, one obtains amorphous phthalocyanine layers which present an excellent sensitivity because, due to the low temperature, the molecules deposit in a random manner. The cooling of the substrate results in the fact that the kinetic energy of the phthalocyanine molecules, originating from the evaporation of the phthalocyanine, is practically immediately taken up when the molecules of the phthalocyanine enter into contact with the substrate. The molecules remain in this way blocked in their arrival position and do not reorient in a crystalline network. The semiconductor element thus manufactured presents donor and acceptor sites, which enable fire gases produced with or without flames to reach a site that is capable of detecting them.
A preferred embodiment of a method according to the invention is characterized in that the vacuum evaporation is realized by bringing the phthalocyanine source at a temperature of approximately 350xc2x0 C. in a vacuum of approximately 10xe2x88x924 during a period of time of approximately 10 minutes. The temperature essentially determines the speed at which the phthalocyanine will sublime and thus determine the deposition rate on the substrate. That speed also conditions the crystallization of the film. By adjusting the temperature, the crystallization speed is thus determined.
Another preferred embodiment of a method according to the invention is characterized in that the phthalocyanine layer is a mixture of different metallophthalocyanines. The small difference in form and dimension of the phthalocyanine molecules of, for example copper and cobalt, is sufficient to destablize the network and give rise to a polycrystalline growth.
Still another preferred embodiment of a method according to the invention is characterized in that for the substrate, an alumina substrate obtained by sintering alumina powders belonging to different granulometric classes is used. The use of powders belonging to different granulometric classes enables achievement a non planar surface of the substrate.
Yet another preferred embodiment of a method according to the invention is characterized in that the substrate used is a substrate based on sintered powders, which substrate is impregnated, before the application of the phthalocyanine, with a water-repellent substance that is capable of polymerizing in the residual pores that are present after sintering. The use of substrates based on sintered powders such as alumina, presents the drawback of having several pores and interstices between the alumina grains. When the humidity of the ambient atmosphere increases up to 70% of relative humidity, those pores can be the seat of capillary condensation. Under these circumstances, the electrical resistance of the alumina substrate diminishes considerably so as to become lower than the one of the sensitive layer. Those two resistances being connected in parallel, it will be the weakest, in this case the one of the substrate, which will become determinant. In order to avoid this effect, the substrate is first impregnated with a water-repellent substance capable of polymerizing in the pores of the sintered alumina. A later capillary condensation will be thus avoided.
Yet another preferred embodiment of a method according to the invention is characterized in that the substrate is a silicon substrate covered by an insulating layer of which the surface has been rendered porous and rough. The intrinsic resistive value of the layers with which the silicon can be covered makes that substrate particularly suitable.